I saw this movie shortly before I entered on duty with the BP in 1982. In those days we were allowed to carry any American made 3.5" to 4" barrel length .357 revolver. I saw all kinds of different models when I was in, I carried a privately owned Model 28 after the academy and after getting issued a stainless Ruger in the academy. We had 19's, 66's, Rugers, Colt Border Patrols (these were getting pretty rare while I was in, but a few were still floating around), and a few N frames. I even had a coworker who hard chromed a Python and carried that for awhile. I told him that that was a criminal offense in some jurisidictions! Shortly before I entered on duty the old timers were telling me that 1 of the agents was killed by his wife with his carry gun, a 3.5" Model 27, after he came home muy borracho 1 night and decided to tune her up, and she punched his 10 ring with his gun.
So basically, it's liable that they were carrying a prewar 3.5". BTW I heard that the BP didn't like they way they were portrayed in that movie and refused to cooperate with the production. Other border movies that were good for "after hours training with adult beverages" were "Borderline" with Charles Bronson and "The Border Incident".